Electrical power is traditionally generated by a thermodynamic process. Heat, for example, may be generated by burning oil in a boiler to superheat pressurized water. The superheated water is expanded into pressurized steam that mechanically rotates a turbine. Rotation of the rotor windings of an electric generator rotor connected to the turbine through an appropriate magnetic field generates electrical power.
Conventional electrical power generation uses a thermal/mechanical process the efficiency of which is limited by the Carnot cycle. The Carnot cycle mandates that, even under ideal conditions, a heat engine cannot convert all the heat energy supplied to it into mechanical energy, and therefore a significant portion of the heat energy is rejected. In the Carnot cycle, an engine accepts heat energy from a high temperature source, converts part of the heat energy into mechanical work, and rejects the remainder of the heat energy to a low temperature heat sink. The rejected heat energy causes a loss in efficiency.
A different process for generating electricity utilizes a solid oxide fuel cell. Electrical power results from the direct conversion of the energy released by a chemical reaction into electrical power, rather than a thermal/mechanical process. As a result, solid oxide fuel cells are not limited in efficiency by the Carnot cycle and highly efficient electrical power generation is theoretically possible.
One solid oxide fuel cell is disclosed in U.S. Pat. No. 5,413,879 to Domeracki et al. that is incorporated by reference in its entirety herein. The patent discloses a solid oxide fuel cell having a gas tight ceramic membrane that separates an air chamber from a fuel chamber. The ceramic membrane is typically a three layer composite having a gas tight core portion formed from a ceramic membrane material, such as yttria-stabilize zirconia, that selectively transports oxygen ions by diffusion. A portion of the surface of the ceramic membrane in contact with air is coated with an electrode that may be made of strontium-doped lanthanum manganite. A portion of the opposing surface of the ceramic membrane in contact with fuel is a fuel electrode that may be a nickel-zirconia cermet. Interconnects are provided on both electrodes which permit connecting several electrical cells in series or parallel and withdraw an electric current generated by the ion flux. Suitable solid fuel cells are disclosed in U.S. Pat. No. 4,490,444 (Isenberg) and U.S. Pat. No. 4,728,584 (Isenberg), each of which is hereby incorporated by reference in its entirety.
Hot air contacts the air electrode and oxygen is separated from the air by ion transport through the ceramic membrane to the surface of the fuel electrode. A gaseous fuel, typically a light hydrocarbon such as natural gas or carbon monoxide, contacts the fuel electrode surface and exothermally reacts with the oxygen ions to produce electricity and heat as the result of internal losses. Exiting the fuel cell are a hot partially oxygen depleted gas from the cathode or retentate side and reaction or combustion products from the anode or permeate side.
Electric power generating systems using solid oxide fuel cells are limited in attainable efficiencies due to several factors including: (1) internal electrical losses primarily in the electrodes, (2) the high temperature in the range of about 700.degree. C. to about 1,000.degree. C. to which air must be heated; and (3) the fact that only a portion of the oxygen contained within the hot air, typically on the order of between 20% to 30% by volume of the oxygen available, is transported through the ceramic membrane for reaction with the gaseous fuel. The remainder of the oxygen is discharged in the retentate stream exiting the air chamber. Part of the energy added to the retentate and permeate streams is lost as the result of pressure drop and limited effectiveness of optional recuperative heat exchangers.
U.S. Pat. No. 5,413,879 (Domeracki) discloses combining the reaction products from chemical reactions in the fuel chamber with the hot gas retentate from the air chamber and reacting it with additional fuel in a combustor to further elevate the temperature of the mixture. The hot mixture heats a compressed gas which is used to drive a turbine.
Several types of ion transport membranes are disclosed in U.S. Pat. No. 5,733,435 (Prassad et al.) For membranes that exhibit only ionic conductivity, external electrodes are placed on the surfaces of the membrane and the electron current is returned by an external circuit. In mixed conducting membranes, electrons are transported to the cathode side internally, thus completing a circuit and obviating the need for external electrodes in a pressure-driven mode. Dual phase conductors, in which an ionic conductor is mixed with an electronic conductor, may also be used for the same application.
U.S. Pat. No. 4,793,904 to Mazanec et al., that is incorporated by reference in its entirety herein, discloses an ion transport membrane coated on both sides with an electrically conductive layer. An oxygen-containing gas contacts one side of the membrane. Oxygen ions are transported through the membrane to the other side where the ions react with methane or similar hydrocarbons to form syngas. The electrons released by the oxygen ions flow from the conductive layer to external wires and may be utilized to generate electricity.
In a mixed conductor type membrane, the membrane has the ability to selectively transport both oxygen ions and electrons. It is not necessary to provide an external electric field for the removal of the electrons released by the oxygen ions. U.S. Pat. No. 5,306,411 to Mazanec et al., that is incorporated by reference in its entirety herein, discloses application of mixed conductor and dual phase conductor membranes.
The membrane comprises either "single phase" mixed metal oxides having a perovskite structure with both ion- and electron conductive properties or a multi-phase mixture of an electron-conductive phase and an ion conductive phase. The oxygen ion transport is disclosed as being useful to form syngas and to remediate flue gases such as NO.sub.x and SO.sub.x.
U.S. Pat. No. 5,516,359 to Kang et al. discloses a ceramic ion transport membrane integrated with a high temperature process in which heat is utilized effectively for the operation of both the membrane and the high temperature process. Hot compressed air contacts with an oxygen selective ion transport membrane and a portion of the oxygen contained within the air is transported through the membrane and removed as a product gas. The oxygen depleted residual gas is combined with a gaseous fuel and reacted to generate a high temperature gas useful to drive a turbine that typically drives an air compressor and a generator for electrical power generation.
There remains, however, a need for a process that integrates ion transport reactors with the more efficient solid oxide fuel cell for the generation of one or more product gases and electric power to realize an improvement in efficiency.